The present invention relates to a developing device, a developing method and an image forming apparatus.
First, a conventional developing device will be described in which a toner carrier is rotatably supported on a housing of the developing device. Further, a limiting member for limiting the thickness of a toner layer on the toner carrier is disposed so as to face the toner carrier. The limiting member for limiting the toner layer thickness forms the toner into a thin layer over the surface of the toner carrier. This limiting member comprises a rubber holding plate and a frictional limiting plate, such as a rubber plate, attached to the front end of the rubber holding plate. The holding member of the limiting member is secured to the housing by screws fitted into threaded holes in the housing, so that the frictional limiting plate comes into contact with the toner carrier.
Japanese Patent Laid-open No. 309080/1989 discloses a construction in which the toner carrier is rotatably supported on the housing by a support shaft, and in which the limiting member for limiting the toner layer thickness is positioned by taking the support shaft of the toner carrier as a reference. Japanese Patent Laid-Open No. 89869/1980 discloses a means for connecting together and supporting a toner carrier and a limiting member for limiting the developer layer thickness and defines the linear expansion coefficients necessary to improve the positioning precision of these elements.
Conventional developing devices incorporate various provisions to enable the toner carrier and the limiting member for limiting the toner layer thickness to be positioned precisely to form a thin layer uniformly. They also include a variety of improvements to deal with leakage of toner from the developer case. There has been a demand in recent years for further uniformity in image quality to enhance the image quality and the resolution of the developing device. This in turn requires improved performances of the toner carrier and the limiting member for limiting the toner layer thickness and further improvements in the measures taken to prevent toner leakage.
Conventional developing devices have both the toner carrier and the limiting member for limiting the toner layer thickness secured to the housing by a support plate in their final configuration.
In such a developing device, the toner in the form of fine powder is sealed in the housing. To prevent leakage of the toner to the outside of the housing, the upper and lower cases of the housing need to be closely held together. For this purpose, the upper and lower cases of the housing are fused together by ultrasonic welding to keep the housing airtight. Alternatively, a packing is used when the upper and the lower cases are hermetically held together by screws.
In these cases, the portion, where the limiting members for limiting the toner carrier and the toner layer thicknesses are secured to the housing through a support plate or the like, is formed with an opening. Therefore, when the upper case is pushed from above, the case may be deformed, producing a negatively activated pressure corresponding to the deformed volume of the case, thereby sending toner flying along with air entering from the opening where the limiting members for limiting the toner carrier and the toner layer thicknesses are secured to the housing through the support plate. Then, when the developing device is mounted to or dismounted from the color image forming apparatus, the dispersed toner may fall out of the device. Also, during image development, the toner may fall from the toner carrier and the limiting member for limiting the toner layer thickness due to the rotation of the parts.